1. Field of the Invention
This invention relates to circuitry for controlling the value of currents and, more particularly, to methods and apparatus for providing improved current mirroring circuitry.
2. History of the Prior Art
In electronic circuitry, it is often desirable to provide a current which has a value identical to that of a current through some other circuit device not in series with the first device. This is typically accomplished through the use of a current mirroring circuit. By maintaining the voltage levels equal at gate and source terminals of a pair of field effect transistor (FET) devices operating in saturation, the current through the devices may be kept equal. However, there are situations in which circuit conditions tend to vary during use; and it is necessary to maintain the current equality in the face of these variations. To accomplish this, a negative feedback scheme has been used as a part of the current mirror circuitry to adjust the currents in response to current variations caused by noise, changes in ambient temperatures, or the like.
A problem with such a feedback scheme is that it produces an inherent mismatch in the drain voltages of the mirroring transistors and a degradation of the accuracy of the mirror. Sometimes more accuracy of current equality is necessary than is provided by the standard negative feedback arrangement.
It is desirable to provide circuitry which will produce a more accurate self-compensating arrangement for current mirroring.
Another problem encountered with current mirroring arrangements which use negative feedback for compensation is that such systems must be designed to operate with a particular supply voltages. For example, some portable computers are designed to perform with either five volt or 3.5 volt power supplies. To function with one system or the other, the current mirroring arrangements must be designed for the particular range. It is possible for a current mirroring arrangement which uses negative feedback to perform well in one range but improperly outside that designed range. Often circuit boards must be designed so that they may be used with more than one range of supply voltages. This has eliminated the ability to use self-compensating current mirroring arrangements.
It is desirable for a current mirror circuit to be able to mirror currents very accurately within its designed range of operation yet continue to mirror current with acceptable accuracy outside the designed range of operation.